Catalyst structural types

Several types of catalysis are described by terms denoting the catalyst structure or function. 

Zeolite, or more properly, faujasite, is the key ingredient of the FCC catalyst. It provides product selectivity and much of the catalytic activity. The catalyst s performance largely depends on the nature and quality of the zeolite. Understanding the zeolite structure, types, cracking mechanism, and properties is essential in choosing the right catalyst to produce the desired yields. 

The reaction of tert-alkyl Grignard reagents with carboxylic acid chlorides in the presence of a copper catalyst provides ieri-alkyl ketones in substantially lower yields than those reported here.4,14 The simplicity and mildness of experimental conditions and isolation procedure, the diversity of substrate structural type, and the functional group selectivity of these mixed organocuprate reagents render them very useful for conversion of carboxylic acid chlorides to the corresponding secondary and tertiary alkyl ketones.15... 

The results of the EXAFS studies on supported bimetallic catalysts have provided excellent confirmation of earlier conclusions (21-24) regarding the existence of bimetallic clusters in these catalysts. Moreover, major structural features of bimetallic clusters deduced from chemisorption and catalytic data (21-24), or anticipated from considerations of the miscibility or surface energies of the components (13-15), received additional support from the EXAFS data. From another point of view, it can also be said that the bimetallic catalyst systems provided a critical test of the EXAFS method for investigations of catalyst structure (17). The application of EXAFS in conjunction with studies employing ( mical probes and other types of physical probes was an important feature of the work (25). 

When supported complexes are the catalysts, two types of ionic solid were used zeolites and clays. The structures of these solids (microporous and lamellar respectively) help to improve the stability of the complex catalyst under the reaction conditions by preventing the catalytic species from undergoing dimerization or aggregation, both phenomena which are known to be deactivating. In some cases, the pore walls can tune the selectivity of the reaction by steric effects. The strong similarities of zeolites with the protein portion of natural enzymes was emphasized by Herron.20 The protein protects the active site from side reactions, sieves the substrate molecules, and provides a stereochemically demanding void. Metal complexes have been encapsulated in zeolites, successfully mimicking metalloenzymes for oxidation reactions. Two methods of synthesis of such encapsulated/intercalated complexes have been tested, as follows. 

Ligand 73 was prepared directly from a single enantiomer of the corresponding naphthol of QUINAP 60, an early intermediate in the original synthesis, and both enantiomers of BINOL. Application in hydroboration found that, in practice, only one of the cationic rhodium complexes of the diastereomeric pair proved effective, (aA, A)-73. While (aA, A)-73 gave 68% ee for the hydroboration of styrene (70% yield), the diastereomer (aA, R)-73 afforded the product alcohol after oxidation with an attenuated 2% ee (55% yield) and the same trend was apparent in the hydroboration of electron-poor vinylarenes. Indeed, even with (aA, A)-73, the asymmetries induced were very modest (31-51% ee). The hydroboration pre-catalyst was examined in the presence of catecholborane 1 at low temperatures and binuclear reactive intermediates were identified. However, when similar experiments were conducted with QUINAP 60, no intermediates of the same structural type were found.100... 

The patent literature discloses alkylation performances of several additional structure types. A Mobil patent (182) describes the use of VTM-A, a pillared titanosilicate of the MCM-27 family. The catalyst produced about 80 wt% of octanes under relatively mild conditions (OSV = 0.05 h 1, P/O ratio = 20). A number of patents describe the use of MCM-36. MCM-49, which is closely related to MCM-22, has also been tested as an alkylation catalyst. In general,... 

Given the complexity of the pore structure in high-surface-area catalysts, six types of adsorption isotherms have been identified according to a classification advanced by IUPAC 145-481. Out of these six, only four are usually found in catalysis ... 

The nomenclature of zeolites is rather arbitrary and follows no obvious rules because every producer of synthetic zeolites uses his/her own acronyms for the materials. However, as mentioned before, at least the structure types of the different zeolites have a unique code. For example, FAU represents Faujasite-type zeolites, LTA Linde Type A zeolites, MFI Mobile Five, and BEA Zeolite Beta. The structure commission of the International Zeolite Association (IZA) is the committee granting the respective three-letter codes [4], Some typical zeolites, which are of importance as catalysts in petrochemistry, will be described in the following sections. 

Merlen, E., Alario, F., Ferrer, N., and Martin, O. (2005) Catalyst comprising at least one zeolite with structure type NES and rhenium, and its use for transalkylation of aromatic hydrocarbons. US Patent 6864400. 

Because BaO/NaX zeolite catalysts exhibited the best performance, further investigations have been carried out recently to characterize the oxidative methylation of toluene catalyzed by BaO-modified X- and Y-zeolites, mordenite, ZSM-5, sil-icalite, and ALPO4-5 (230). The authors found that activity and basicity of BaO-modified zeolites and zeolite-like catalysts depend on both the structural type and composition. Thus, for samples of the same structural type (BaO/NaX zeolite. 

The advantages of shape selective catalysis are alreacfy ejq)loited in a number of industrial processes [11-14]. Astonishingly, virtually all these processes rely on a single structural type of catalyst, viz. zeolite ZSM-5 in various modifications, or its titanium containing analogue TS-1 [15]. It is, moreover, noteworthy that many of these processes convert and/or produce mononuclear aromatic compounds. It is not surprising, therefore, that a vast scientific literature exists on shape selective reactions of benzene derivatives in zeolite ZSM-5. 

Catalytic reactions involve a complex set of related and linked events. Both the catalyst structure and the type of reactants, intermediates and products adsorbed on the surface impact the functionality of a catalyst. Therefore, an understanding of the various events must be synthesized with the intrinsic kinetics to produce a reaction mechanism that accurately models the catalyst surface. The goal is to characterize and understand the catalyst surface well enough to predict the performance either for a class of reactions or within the tested range. 

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